The transconductance of semiconductor devices, such as metal oxide semiconductor (MOS) transistors, determines the gain for the device. Transconductance is a property of certain electronic components. Conductance is the reciprocal of resistance. Transconductance is the ratio of the current change at an output port to a voltage change at an input port.
The transconductance is generally proportional to hole or electron mobility, at least for low drain voltages. However, as device sizes are reduced, the fields in the channel increase and dopant impurity levels can increase. Both impact carrier mobility and tend to lower transconductance. Furthermore, increasing the electric field in the channel can result in velocity saturation, thereby limiting the current and transconductance.